DESCRIPTION (Abstract verbatim): The long-term goal of this study is to understand the molecular and genetic control of oligodendrocyte specification and differentiation. Recently, three Nkx homeobox genes, Nkx-6.1, Nkx-6.2 and Nkx-2.2, have been identified that are specifically expressed in the oligodendrocyte precursor cells in the ventral spinal cord. In addition, expression of these three Nkx genes can also be detected in the differentiating or mature oligodendrocytes at late stages of embryogenesis. Thus, it has been hypothesized that the Nkx genes play important roles in controlling the specification, differentiation or maturation of oligodendrocytes. Four specific aims are proposed here to test this hypothesis and to systematically characterize the function of the Nkx genes in the control of oligodendrocyte development. Aim 1 is to characterize the oligodendrocyte phenotypes in the Nkx-2.2 mutant mice. Aim 2 is to investigate the effects of the Nkx-6.2 mutation on oligodendrocyte differentiation. Aim 3 is to study the oligodendrocyte development in the Nkx-2.2 and Nkx-6.2 double mutants to investigate their possible redundant role in controlling the oligodendrocyte development. Aim 4 is to test the effects of Nkx-6.1, Nkx-6.2 and Nkx-6.2 ectopic expression on oligodendrocyte differentiation by overexpressing these genes in the dorsal spinal cord of chicken embryos using the replication-competent avian retrovirus as a gene delivery system. To test the possibility that simultaneous expression of three Nkx genes is required for oligodendrocyte induction, we will study the oligodendrocyte development in the Pax-6 mutants, in which the Nkx-2.2 expression is dorsally expanded within the Nkx-6.1+ and Nkx-6.2+ domain. Thus, the ventricular precursor cells coexpressing Nkx-2.2 and Nkx-6.1, Nkx-6.2 is also dorsally expanded in the Pax-6 mutants and oligodendrocyte precursor domain might be according extended dorsally. Results derived from the proposed studies will significantly enhance our understanding of the genetic circuitry governing the early specification and differentiation of oligodendrocytes, and may provide theoretic basis for design of novel therapeutic approaches for prevention and treatment of motor neuron and oligodendrocyte atrophies.